The conventional technology for producing dimethyl ether (DME) can be split into two broad categories.
In the first category, DME is produced by dehydrating methanol into DME and byproduct water in a dehydration unit containing a methanol dehydration catalyst where the methanol stream is first produced by converting syngas (i.e. a synthesis gas stream containing hydrogen and carbon monoxide) to methanol in a gas phase reactor containing a methanol synthesis catalyst.
In the second category, the syngas conversion step and methanol dehydration step are combined in a single unit comprising a slurry bubble column reactor (SBCR) containing a bifunctional catalyst system having a methanol synthesis functionality and a methanol dehydration functionality. See for example U.S Pat. No. 5,218,003 assigned to Air Products and Chemicals, Inc.
A concern with the first category is that the gas phase synthesis of methanol produces a relatively high water content crude methanol stream, thereby requiring an intervening water removal unit (i.e. between methanol unit and dehydration unit) and/or an incremental increase in the water that must be processed by the dehydration unit.
One objective of the present invention is to address this concern by selecting a methanol synthesis process that produces a relatively low water content as compared to the gas phase methanol synthesis process. A second objective of the present invention is to provide an alternative to the second category of DME production where it is desired to use a separate methanol synthesis catalyst and a separate methanol dehydration catalyst.
U.S Pat. No. 4,031,123 assigned to Air Products and Chemicals, Inc. (Air Products) teaches a process to produce methanol by converting syngas to methanol in a SBCR containing a methanol synthesis catalyst. Air Products further teaches that while some water is formed in the reaction of the SBCR methanol process, the quantity is small (usually less than 3% of total products). In addition to producing a relatively low water content methanol stream, other advantages of Air Products"" SBCR methanol unit as compared to a gas phase methanol unit include the ability to dissipate heat rapidly, the flexibility to process syngas compositions (especially carbon-rich compositions) originating from a wide variety of feedstocks (e.g. coal, natural gas, petroleum coke, waste materials, etc), and even the flexibility to handle fluctuating syngas compositions.
U.S Pat. No. 5,750,799 assigned to Starchem, Inc. (Starchem) teaches a process to produce DME by dehydrating methanol into DME and byproduct water in a dehydration unit containing a methanol dehydration catalyst. Starchem, noting that the methanol feed to the dehydration unit desirably should be relatively free of any water content, teaches methanol feed streams having water contents ranging from about 1 weight % up to about 18 weight %.
Heretofore, the prior art has not captured the advantages of coupling a low water producing SBCR methanol unit to a methanol dehydration unit for DME production.
The present invention is a process for producing DME by dehydrating the effluent stream from a methanol reactor where the methanol reactor is a slurry bubble column reactor (SBCR) containing a methanol synthesis catalyst that converts a synthesis gas stream comprising hydrogen and carbon monoxide into an effluent stream comprising methanol. A key to the process is that the SBCR methanol can produce a relatively low water content methanol stream, thereby avoiding an intervening water removal unit between the SBCR methanol unit and the dehydration unit, and/or an incremental increase in the water that must be processed by the dehydration unit.